James,
A lengthy article on how to do just this is in Military Vehicles Magazine, issue 92.

The short form is there are date tags on the axles, transmission, and transfer case. Look at them, whatever date is most common is probably accurate. There is a tag on the engine to, but the engines have almost always been exchanged, so don't trust it.

Also, the tag on the transfer case almost always has an abreviation of the truck builder's name on it.